This invention relates to the finishing, i.e., final processing, of elastomeric polymers that are normally produced in crumb form, which typically has a fine particle size. More particularly, the invention relates to a process for converting difficult to process sticky polymer crumb into an easy to process pellet form without significant polymer degradation.
Elastomeric polymers of styrene and butadiene or isoprene are anionically polymerized in an organic solvent. Such polymers are also often hydrogenated while in the solvent. The final step in the production of these polymers requires removing the solvent from the polymer/solvent mixture/slurry/suspension, usually referred to as the polymer cement, to produce dry material which can be packaged. This final processing step is often referred to as xe2x80x9cfinishingxe2x80x9d the polymer. These polymers are generally produced as a crumb that is sometimes difficult to handle and is many times undesirably sticky as well. Problems associated with the adhesive nature of this sticky material put limitations on whether it can be realistically or profitably manufactured.
Even when it is realistically possible to manufacture these products, the product""s form can be difficult for endusers to handle and put to its desired use. Such products are often sold as crumb in bags. This product form is difficult for some users to handle. Crumb particle size is often fine and it tends to coat equipment, particularly in the case of sticky grades, creating mess and waste. Some products block in the bags, forming a 30 to 40 lb. xe2x80x9cpillowxe2x80x9d of polymer. Bags of polymer must be cut open by hand and the blocked material has to be fed into a mechanical grinder prior to mixing with other ingredients.
Many polymers, especially thermoplastic but non-elastomeric polymers, are conveniently manufactured in pellet form. This form is very easy to handle and agglomeration problems can be easily solved by dusting the polymer with anti-stick agents. Pellets of these commercial thermoplastic polymers are formed with melt extruders, often twin screw extruders, which carry out their function by melting the polymer and extruding it through a die where it is chopped into small pellets. Many of the polymers of this invention are high molecular weight materials and highly elastic materials. When these polymers are processed in twin screw melt extruders, they tend to generate enough shear heat to cause significant degradation. Degradation causes the polymer properties to suffer and is a significant disadvantage.
It is clear therefore that it would be highly advantageous to be able to finish the sticky elastomeric polymers of this invention in such a manner that they could be produced in pellet form. It would be most advantageous that this process be able to be carried out without significant polymer degradation.
This invention solves the problems discussed above. Elastomeric anionic polymers of styrene and butadiene or isoprene, including polyisoprene star polymers, are anionically polymerized as in the past. This processing may also incorporate hydrogenation if desired. The polymer is produced in crumb form.
The dried polymer crumb is then converted to pellets via solid state extrusion. The polymer crumb is extruded in a single screw extruder which has a longitudinally grooved barrel and has pins extending into the barrel transverse to the flow of the polymer. The extruder has a length to diameter (L/D) ratio of 10:1 or less, preferably 8:1 or less, and is operated at 30 to 100 rpm, preferably 40 to 60 rpm. The temperature of the polymer in the extruder must be sufficient to agglomerate or melt the polymer but the temperature should not exceed the degradation temperature of the polymer. Preferably, the solid state extrusion is carried out at 200xc2x0 C. or less and most preferably 160xc2x0 C. or less.
It is necessary to use a single screw extruder in this solid state extrusion process in order to minimize shearing of the polymer. Excessive shearing can cause an undesirable increase in the temperature of the polymer which, as discussed above, can cause significant degradation. Twin screw extruders increase the shearing of the polymer and thus they may not be used in the present invention.
In this process, sufficient mechanical heat is generated by the polymer extrusion without auxiliary heating of the equipment or preheating of the crumb being necessary. Sufficient heat must be generated in order to agglomerate or melt the polymer sufficiently so that it can be extruded and then cut into pellets. By agglomerate, we mean that the polymer is soft enough and sticky enough to stick together but has not yet passed through the glass transition temperature which is the point at which the polymer melts.
Polymers of the type described herein are known to degrade at temperatures of 300xc2x0 C. and higher so it is important that the temperature in the single screw extruder be less than that. However, it is possible that higher localized temperatures can occur in the extruder so it is highly preferred that the temperature in the extruder be 200xc2x0 C. or less. It is most preferred that the temperature be 160xc2x0 C. or less to minimize localized temperature peaks which can cause degradation of the polymer at those locations.
The use of a single screw (as opposed to twin screw) is necessary to get agglomeration without high temperature but it is important that sufficient mixing of the polymer occur. In order to make certain that this occurs, the barrel of the single screw extruder has longitudinal grooves and pins extending into the barrel transverse to the flow of the polymer. These features increase the mixing without dramatically increasing the shearing of the polymer.
The longer the polymer is processed in the extruder, the more likely it is that degradation of the polymer will occur. Thus, it is preferred that long extruders not be used. It is preferred that the length to diameter (L/D) ratio be 10:1 or less, preferably 8:1 or less, most preferably about 4:1.
In order to obtain sufficient mixing, the speed of the extruder screw should be from 30 to 100 rpm for extruders with an L/D ratio of from 2:1 to 10:1. If the L/D ratio is smaller, then the speed of the screw can be lower. Again, the goal is to provide sufficient mixing without heating up the polymers to a temperature where it degrades.
The polymers suitable for finishing by the process of this invention include hydrogenated homopolymers and copolymers of diolefins containing from 4 to about 12 carbon atoms, hydrogenated copolymers of one or more conjugated diolefins and one or more monoalkenyl aromatic hydrocarbons containing from 8 to about 16 carbon atoms and the like. The base polymer may be of a star or linear structure. Hydrogenated polymers may be hydrogenated selectively, completely or partially. Hydrogenated polymers of conjugated diolefins and copolymers of conjugated diolefins and monoalkenyl arenes are preferably hydrogenated such that greater than 90% of the initial ethylenic unsaturation is removed by hydrogenation. Preferably, the hydrogenated polymers are substantially free of ethylenic unsaturation.
Selective hydrogenation refers to processes that hydrogenate a substantial portion of the ethylenic unsaturation and a substantial portion of the initial aromatic unsaturation is left unhydrogenated. As used herein, a hydrocarbon polymer substantially free of ethylenic unsaturation will be a hydrocarbon polymer containing, on average, less than about 10 carbon-carbon ethylenic double bonds per polymer chain. Polymers containing more than this amount of ethylenic unsaturation will, under certain conditions, exhibit excessive crosslinking during a functionalization reaction when the finctionalization is completed in a blending apparatus capable of imparting high mechanical shear.
Useful hydrocarbon polymers include those prepared in bulk, suspension, solution or emulsion. As is well known, polymerization of monomers to produce hydrocarbon polymers may be accomplished using free-radical, cationic and anionic initiators or polymerization catalysts.
A wide range of molecular weight polymers can be processed as described herein. In general, the higher the molecular weight of the polymer, the more likely it is that degradation of the polymer will occur in conventional melt processing. Thus, this invention is especially advantageous for higher molecular weight polymers. In general, polymers with weight average molecular weights of between about 100,000 and about 1,200,000 may be processed according to this process
The weight average molecular weights, as used herein, for linear anionic polymers refers to the weight average molecular weight as measured by Gel Permeation Chromatograph (xe2x80x9cGPCxe2x80x9d) with a polystyrene standard. For star polymers, the weight average molecular weights are determined by light scattering techniques.